Let me qualify.
A run and break manoeuvre is perfectly safe at an uncontrolled airfield if either (a) there is no circuit traffic, or (b) all the circuit traffic knows what a run and break means.
(a) is fine if you own the airfield, for example, or have amazing eyesight, or can guarantee that there is no NORDO traffic at the airfield. But it is hard to be 100% certain.
(b) is fine if the airfield is a military one (out of hours), or you know everybody who uses the airfield (e.g. you own it), but dodgy otherwise as it's not taught in the PPL syllabus.
There is nothing inherently dangerous about the manoeuvre, but it is not safe if there is other circuit traffic who do not know what to expect.
Tim